A PVD process may be used to deposit a target material such as titanium or titanium nitride onto a semiconductor wafer or other article. In a typical PVD apparatus, the target material and the article to be coated are disposed in a vacuum chamber containing an inert gas such as argon. An electrically conductive shield is generally provided circumscribing the target and the article. Upon application of a sufficiently high electrical potential between the cathodic target and the anodic shield, the inert gas ionizes, forming a high energy plasma. Relatively high mass, positively charged plasma ions impinge the negatively charged target with sufficient energy to expel atoms from the target, forming a physical vapor. The expelled or sputtered target atoms follow various ejection trajectories due to initial direction and subsequent collision with other sputtered target atoms, plasma ions, and inert gas molecules. The sputtered particle vapor fills the chamber and the particles eventually deposit on exposed surfaces of the article to be coated. A typical PVD apparatus or sputter deposition device is described in U.S. Pat. No. 4,824,544 issued to Mikalesen et al., the disclosure of which is herein incorporated by reference in its entirety.
Semiconductor wafer surfaces to be coated, while macroscopically planar, include numerous microscopic surface features including apertures such as contact openings and vias, raised portions such as mesas, and depressed channels. To provide uniform coating of these surfaces, a collimator filter may be disposed between the target and the semiconductor wafer. The collimator structure forms a plurality of passages or apertures passing therethrough, the apertures having respective centerlines oriented substantially normally to the wafer surface. Accordingly, solely sputtered atoms traveling along normal and near normal trajectories pass through the collimator and are deposited on the wafer, while those traveling along oblique trajectories greater than a critical angle are deposited on the collimator. The angular range of trajectories which pass through the collimator is predetermined by manufacturing the collimator passages to a selected height and cross-sectional configuration and size. Collimators are especially useful for ensuring coating of bottom and side walls of wafer apertures and channels having a large aspect ratio since they prevent excessive buildup of deposited material at the aperture and channel openings which can bridge the openings, leaving voids therebelow. Aspect ratio is the ratio of aperture diameter or channel width to depth of the feature.
Conventional collimators may be made by individually machining a plurality of apertures through a solid disk of material. The apertures are typically holes of cylindrical cross-section machined by conventional processes such as by drilling or by non-conventional metal removal techniques such as by electrical discharge machining ("EDM"), wire EDM, abrasive water jet cutting, and laser machining. Machined collimators produce a relatively large amount of waste material, tend to be relatively heavy due to required minimum wall thickness between adjacent apertures, and are costly to produce due to the extensive machining operation. Further, the ratio of collimator passage open area to unmachined area between the passages is relatively low, resulting in excessive blockage of sputtered atoms traveling along otherwise acceptable normal and near normal trajectories. Alternatively, collimators may be manufactured by close packing of a plurality of discrete tubes or by creating a grid of square or rectangular passages by using a series of intersecting plates as disclosed by Mikalesen et al. Such collimators, however, also tend to be relatively heavy, costly to produce, and generate imprecise collimator passage geometries.
Another method of manufacturing collimators includes corrugating thin strips of metal foil and attaching the strips to one another by welding at nodes where adjacent foil strip wall sections contact one another. While such welded collimators may be lighter and produce less blockage than machined collimators, a contamination problem exists inherently due to the manufacturing process.
For proper PVD processing of semiconductor wafers, the PVD apparatus components, the semiconductor wafer, and the vacuum chamber inert gas must be substantially free from contaminants. Contaminants in the vacuum chamber, either in the form of foreign particulate or gaseous matter, may be deposited on the wafer and on the chamber component surfaces during the PVD process. Such contaminants render a certain percentage of the devices on the wafer and subsequently processed wafers unusable, negatively impacting process yields and increasing cost per device.
Welded collimators produce inherently both bonded and unbonded areas in the nodes. Further, welded collimators produce gaps or voids in the nodes due to locally irregular geometries of the wall sections which are neither perfectly flat nor perfectly mated. Because of these voids, foreign matter introduced during the manufacturing process may become trapped in the nodes. This foreign matter, as well as a buildup of target material on the collimator, can be released or flake off during PVD processing due to sliding, flexing, and other relative motion of the imperfectly bonded wall sections caused by uneven heating and thermal cycling of the collimator. Additionally, periodically during the useful life of the collimator, the collimator is cleaned of built up target material by submersion in an acid etch bath or similar chemical cleaning solution, typically followed by rinsing in deionized water. Thereafter, the collimator is heated to anneal the structure and remove any residual internal stresses resulting from PVD process thermal cycling. Residual contaminants typically remain trapped in the voids in the nodes. Once the welded collimator is placed in the PVD vacuum chamber and the vacuum chamber is evacuated to as low as about 1.times.10.sup.-8 Torr, these trapped contaminants can be released by a process known as outgassing or offgassing by those skilled in the art. Such outgassing continues for hours and potentially continues for the remaining useful life of the welded collimator. Outgassing is exacerbated during PVD processing due to thermal effects at the elevated temperatures required for PVD processing, which may be on the order of about 600.degree. C. (1100.degree. F.).
Accordingly, there exists a need in the art for a light weight, low cost, low blockage collimator which is not subject to outgassing and which does not generate particulate contaminants under. PVD processing conditions.